The invention relates to an inverter arrangement which comprises two inverter units which comprise an intermediate voltage circuit and phase-specific switch components.
In modern electric motor driven devices inverters are commonly used because of the properties achieved by them, such as good efficiency and accurate controllability of the motor. Inverters provided with intermediate voltage circuits generate short direct voltage pulses which are used as the input voltage of the motor and whose pulse width is changed when needed. By the existing modulation methods the switches of inverters can be controlled so that the desired rotation speed and torque of the motor can be maintained in sudden changes and the full torque can be generated even at zero speed.
The length of the motor cables between the inverter and the motor causes problems as the voltage drops in the cables. Thus the poles of the motor cannot be provided with a voltage as high as that in the output of the inverter. Earlier the problem has been solved e.g. by using a high-voltage motor and a step-up transformer in the output of the inverter. Use of a step-up transformer solves the problem of voltage loss, but causes several other problems which impair the properties and controllability of motor driven devices. Due to the step-up transformer the drive used cannot be DC-magnetized or DC-braked. Furthermore, it is in practice impossible to operate the drive at low speeds.
Inductance of the secondary circuit in the step-up transformer forms LC oscillating circuits with the motor cables and the stray capacitance of the transformer. To attenuate the resonance of the oscillating circuits it is necessary to use a separate attenuator, or the modulation component of the switches of the inverter at the same resonance frequency has to be filtered off using a separate LC low-pass filter. The LC filter makes use of modulation methods based on measuring the motor current impossible because, due to the capacitive currents of, the LC filter, the output current of the inverter is not of the same magnitude as the motor current.
The lower the speed is at which the full-load torque is to be obtained from the motor, the greater the voltage increase of the step-up transformer used has to be. In practice the voltage increase must be greater than in conventional transformers because a certain saturation margin has too be reserved for the transformer. Due to saturation margin the transformer must be larger, which in turn increases the costs.
The object of this invention is to provide an arrangement to eliminate the above-mentioned disadvantages and to control the motor even in connection with long cables reliably employing all the modulation techniques used for short cables. This object is achieved by means of the arrangement of the invention which is characterized in that the arrangement also comprises phase-specific one-phase autotransformers the first poles of which are connected to the phase-specific outputs of the first inverter unit and the second poles to the phase-specific outputs of the second inverter unit, and thus the output voltage of the inverter is obtained from the third poles of the autotransformers.
The invention is based on the idea that one-phase autotransformers are used for boosting the output voltage of the inverter, and the two poles of the autotransformers are controlled by two different inverter units while the third pole of the autotransformer generates the output voltage of the inverter. When this arrangement is used, the transformer will not cause resonance problems which would need to be resolved using a separate LC low-pass filter, and thus all properties of a modern inverter can be utilized. Furthermore, autotransformers are considerably smaller than three-phase transformers conventionally used in voltage boosting. Thanks to this, the arrangement of the invention allows to reduce material costs considerably and save space.
In addition to all normal properties of the inverter, such as DC-magnetization an DC-braking, the arrangement of the invention offers new alternative ways of controlling a motor because it allows generation of several different voltage levels which can be connected to the motor in the desired manner. Thanks to the arrangement, the motor can also be controlled with the full torque starting from the zero speed, which has been impossible in prior art step-up transformers.